1. Field of the Invention
The invention relates to wastewater treatment apparatus, and, more particularly, to such apparatus including a drive apparatus for propelling a rotating bridge.
2. Reference to Prior Art
One common type of wastewater treatment apparatus for separating liquids and solids, frequently described as a clarifier, includes a generally circular wastewater treatment tank, a rotatable driven bridge, a sludge moving member supported by the bridge and a drive apparatus. The tank includes a wall having a top surface. The rotatable driven bridge includes one end supported above the center of the tank and an opposite end supported above the tank wall and is supported such that the opposite end of the bridge can move along the tank wall and rotate the bridge about the center. The sludge moving member is supported in the tank by the bridge and is movable in response to movement of the opposite end of the bridge along the tank wall. The drive apparatus for moving the bridge along the tank wall typically includes a frame supported by a pair of drive wheels. The frame supports the end of the bridge for movement along the top of the tank wall. The drive wheels rest on the top surface of the tank wall and support the frame for movement along the tank wall. A drive motor is operably connected to the drive wheels for driving the drive wheels along the top surface of the tank wall.
In such an arrangement, in order to move the bridge the drive wheels must have secure frictional engagement or traction with the top surface of the tank wall. When snow or ice accumulates on either the drive wheels or the top surface of the tank wall, traction declines. When traction declines, movement of the bridge may be interrupted, resulting in malfunction or shutdown of the wastewater treatment apparatus. Thus it is frequently necessary for the drive apparatus to include a cowling extending ahead of the drive wheels to push snow out of the path of the drive wheels. It is also necessary for personnel to scrape ice and snow accumulations from the top wall to improve traction. Spreading sand and salt on the tank wall is also frequently required to improve traction and minimize the rate of ice and snow accumulation.
U.S. Pat. No. 2,418,189 issued to Nordell relates to a clarifier for separating liquids and solids wherein a truss provided with a plurality of scraper blades is supported by two wheels on the upper side of a rail. A pinion engages a rack on the lower edge of the rail to move the truss.
U.S. Patent Reissue No. 23,415 also issued to Nordell relates to a traction drive wherein a wheel riding on the upper surface of a rail moves a rotating bridge. When the force required for moving the bridge is greater than can be secured by the wheel alone, a second wheel can be mounted in contact with the lower edge of the rail.
U.S. Pat. No. 4,859,346 issued to Wood relates to a traction drive which is mounted on a circular support base at ground level below and exterior of the vertical walls of an above ground tank. A continuous rail and rack are mounted on the base, and a wheel and pinion assembly are provided so that the wheel rolls on the rail and the pinion meshes with the rack.
German Patent 2,812,525 relates to a settling tank including a scraper drive trolley having drive wheels with V-shaped tread for straddling the top of a rail and crushing any ice which forms on the rail.